This work, using synthetic DNA as a programmable component to link the nanoparticles,
demonstrates the versatility of DNA-based nanotechnology for the fabrication
of functional classes of materials, particularly optical ones, with possible
applications in solar-energy conversion devices, sensors, and nanoscale circuits.
The research was published online September 29, 2010, in the journal NanoLetters.

BNL scientists used DNA linkers with three binding sites (black “strings”) to connect gold nanoparticles (orange and red spheres) and fluorescent dye molecules (blue spheres) tagged with complementary DNA sequences. These units are self-assembled to form a body-center cubic lattice with nanoparticles at the corners and in the center, and fluorescent dye molecules in between.

“For the first time we have demonstrated a strategy for the assembly
of 3-D, well-defined, optically active structures using DNA encoded components
of different types,” said lead author Oleg Gang of Brookhaven’s
Center for Functional Nanomaterials (CFN).
Like earlier work by Gang and his colleagues, this technique makes use of the
high specificity of binding between complementary strands of DNA to link particles
together in a precise way.

In the current study, the DNA linker molecules had three binding sites. The
two ends of the strands were designed to bind to complementary strands on “plasmonic”
gold nanoparticles — particles in which a particular wavelength of light
induces a collective oscillation of the conductive electrons, leading to strong
absorption of light at that wavelength. The internal part of each DNA linker
was coded to recognize a complementary strand chemically bound to a fluorescent
dye molecule. This setup resulted in the self-assembly of 3-D body centered
cubic crystalline structures with gold nanoparticles located at each corner
of the cube and in the center, with dye molecules at defined positions in between.

The scientists also demonstrated that the assembled structures can be dynamically
tuned by altering the salt concentration of the solution in which they are formed.
Changes in salinity alter the length of the negatively charged DNA molecules,
leading to reversible contraction and expansion of the whole lattice by about
30 percent in length.

“It has long been understood that the distance between metal nanoparticles
and paired dye molecules can affect the optical properties of the latter,”
said Matthew Sfeir, coauthor and an optical scientist at the CFN. In this experiment,
the expansion and contraction of the crystal lattice triggered by the changes
in salt concentration allowed for a dramatic modulation of an optical response:
a three-fold increase in the emission rate of the fluorescent molecules was
observed.

These results were determined using a combination of small angle x-ray scattering
at Brookhaven’s National Synchrotron
Light Source (NSLS) and time-resolved fluorescent methods at the CFN. “This
combination of synchrotron-based structural methods and time-resolved optical
imaging techniques provided invaluable direct insight into the relationship
between the structure and fluorescent properties of these light emitting arrays,”
Gang said.

“Our study tackles important questions about the self-assembly of systems
from components of multiple types. Such systems potentially allow for the modulation
of properties of individual components, and might lead to the emergence of new
behavior due to collective effects. This assembly approach can be applied to
explore such collective behavior of three-dimensional nano-optical arrays —
for example, the influence of the plasmonic lattice on quantum dots.

“An understanding of these interactions would be relevant for developing
novel optical materials for photovoltaic, photocatalysis, computing, and light-emitting
applications. We now have an approach to make these structures and further study
these effects.”

This research was funded by the DOE Office of Science. In addition to Gang
and Sfeir, Huiming Xiong of the CFN and Shanghai Jiao Tong University was a
coauthor on this work.

The Center for Functional Nanomaterials at BNL is one of the five DOE
Nanoscale Science Research Centers, premier national user facilities for
interdisciplinary research at the nanoscale that are supported by the DOE Office
of Science. Together the NSRCs comprise a suite of complementary facilities
that provide researchers with state-of-the-art capabilities to fabricate, process,
characterize and model nanoscale materials, and constitute the largest infrastructure
investment of the National Nanotechnology Initiative. The NSRCs are located
at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia
and Los Alamos national laboratories.